In recent years, optical disks such as CDs (compact disks) and DVDs (digital versatile disks) have come to occupy an important position as information recording media. In devices for reading these optical disks, laser light is emitted along tracks on the optical disk, and the light reflected is detected by an optical pickup mechanism. Recorded data is then read based on changes in the intensity of the reflected light.
Since the data rate for reading from optical disks is extremely high, the light detector for detecting the reflected light is composed of a semiconductor device that uses a PIN photodiode having a high response rate. The weak photoelectric conversion signal generated by the light-receiving part of the semiconductor device is amplified by an amplifier and then output to a subsequent signal-processing circuit. The length of wiring between the light-receiving part and the amplifier is therefore reduced as much as possible in order to maintain the frequency characteristics of the photoelectric conversion signal and to minimize the superposition of noise. The light-receiving part and the circuit part, including the amplifier and the like, are preferably formed on the same semiconductor chip because of these issues and also from the standpoint of reducing the cost of manufacturing the light detector.
FIG. 1 is a schematic cross-sectional view of the vicinity of a light-receiving part of a light detector in which the light-receiving part and the circuit part are positioned adjoining one another on the same semiconductor substrate. The structure of a PIN photodiode (PD) 8 is formed on a semiconductor substrate 2 in a region that corresponds to a light-receiving part 4. Transistors and other circuit elements are formed in a region corresponding to a circuit part 6.
The light detector of FIG. 1 has a two-layer wiring structure. An interlayer insulating film 12, wiring layers 14 and a light-blocking layer 16 that are both composed of aluminum (Al) films, a silicon-oxide film 18, and a silicon-nitride film 20 are layered on the semiconductor substrate 2 as an upper structural layer stack 10. The interlayer insulating film 12 is formed using SOG (spin on glass), BPSG (borophosphosilicate glass), or TEOS (tetra-ethoxy-silane). The silicon-nitride film 20 and the silicon-oxide film 18 together constitute a protective layer for the under layer thereof. The region on the upper structural layer stack 10 that corresponds to the light-receiving part 4 is etched back and an apertured part 30 is formed in that region in order to increase the efficiency of light incidence on the PIN photodiode 8.
A polyimide is further deposited on the upper structural layer stack 10, forming a polyimide film 32. The polyimide film 32 functions as a protective layer for the silicon-nitride film 20. Providing the polyimide film 32 allows the occurrence of cracks on the silicon-nitride film 20 to be minimized, for example, and the moisture resistance to be improved.
FIGS. 2A through 2C are schematic diagrams for describing a conventional method for manufacturing the light detector shown in FIG. 1, showing schematic views of the major steps. The upper structural layer stack 10 is laminated on the semiconductor substrate 2, on which are formed the PD 8 and the like, and the apertured part 30 is formed on the portion that corresponds to the light-receiving part 4 (FIG. 2A).
Once the apertured part 30 has been formed, a polyimide is applied by spin coating, forming a polyimide film 40 (FIG. 2B).
A photoresistive film is applied/formed on the polyimide film 40. The photoresistive film is patterned by photolithography, forming a photoresistive film 42 that covers the circuit part 6 and forms an aperture on the region that corresponds to the light-receiving part 4 (FIG. 2C).
The photoresistive film 42 is then used as an etching mask to etch the polyimide film 40, removing the portion within the apertured part 30. The polyimide film 32 is thereby formed covering the upper surface of the upper structural layer stack 10. The photoresistive film 42 is then removed, whereby the structure shown in FIG. 1 is obtained.
The polyimide film 40 is an organic film, as is the photoresistive film 42. The photoresistive film 42 is therefore also easily etched when the polyimide film 40 is dry etched. Wet etching is therefore preferably used to remove the polyimide film 40 from the apertured part 30.
Problems have been presented in this etching step in that striations and residue from the polyimide film 40 readily arise in the apertured part 30. FIG. 3 is a schematic cross-sectional view that shows the state of the polyimide film 40 after etching. As shown in FIG. 3, for example, a polyimide residue 44 may form in the corners at the bottom of the apertured part 30. In particular, the more the aspect ratio of the apertured part 30 increases; i.e., the more the ratio of depth of the apertured part 30 increases with respect to the width, the more thickly the polyimide film 40 will embed the apertured part 30 and the more prominent the aforementioned problems may become. Polyimide striations and residue within the apertured part 30 have caused problems in light detectors provided with the apertured part 30 that corresponds to the light-receiving part 4 in that inconsistencies occur in the intensity of light incident on the PD 8.
Depending on the characteristics of the surface-covering film, the apertured part may be buried too thickly when the surface-covering film is formed on the upper surface of the upper structural layer stack, which is laminated on a substrate and provided with an apertured part, and thus the usable etching methods are limited. Problems have therefore been presented in that it may be difficult to properly remove the surface-covering film from the apertured part and to selectively form the surface-covering film on the upper surface of the upper structural layer stack.
On the other hand, if the thickness of the applied polyimide or other surface-covering film is too thin, residues and the like may not readily form within the apertured part, but problems will be presented in that the function needed for the surface-covering film; i.e., protecting the upper surface of the upper structural layer stack, will be difficult to maintain.